Many rare species are being pushed towards extinction by exposure to human illnesses, scientists say.

And they believe human encroachment into wild areas is hastening the emergence of new viruses that are passing from animals to people.

They say the increase in what they call "pathogen pollution" between humans and animals is one of the greatest threats to conservation.

And they expect new epidemics similar to Aids will emerge to threaten millions of people.

The scientists reveal their concerns in BBC Radio Four's environment programme Costing the Earth. One, Dr Andrew Cunningham, is a veterinary pathologist at the Zoological Society of London.

Over-exposed

He and his colleagues have found evidence suggesting that the decline in red squirrel numbers in the United Kingdom may have been caused by a pathogen introduced when grey squirrels were brought here from the US.

A pathogen may have attacked the red squirrel

Dr Cunningham is also concerned about the last wild population of Asian lions, which live in a forest area in India.

There are only about 300 animals left there, but Dr Cunningham says they are now exposed to human tuberculosis, because of encroachment into their habitat.

And he says diseases of domestic animals are another risk, with dog viruses threatening India's lions and also its tigers: "Some animals should have a lot less to do with humans than they're being forced to have."

Also in jeopardy are the great apes, according to Professor Robin Dunbar, an evolutionary biologist and primate expert at Liverpool University.

He tells the programme: "Perhaps the best-known case of disease introduced accidentally into apes was the polio outbreak among chimpanzees in Gombe, in Tanzania.

"And there've been cases of measles in gorillas, scabies and yaws. That probably started in primates, came into humans, and now it's being transmitted back into primates.

Extinction threat

"The real root of the problem is that almost all primate species are under serious threat. If there are any great ape populations left in the wild in 20 years' time, we'll have been extremely lucky.

"The mountain gorilla has a total population of about 400 animals. A bout of measles would probably be enough to tip the whole species over the edge into extinction."

The Nipah virus killed a hundred people

Dr Cunningham thinks the risk is seriously underestimated. He says: "Historically we've always thought of the main threats to biodiversity as being habitat destruction and chemical pollution.

"In fact, pathogen pollution appears to be taking over as the main threat.

"As the rate of infectious diseases increases in wildlife, then the chances of humans getting novel diseases also increases.

"There will be terrors out there that are going to catch us by surprise.

More to come

"The Nipah virus in Malaysia last year killed a hundred people. That jumped from bats to pigs to humans, but not from humans to humans.

"If there's another Nipah virus or something similar out there - and there almost certainly is - and it gets into the human species and can be transmitted directly between humans, then we could be facing a pan-global epidemic of a very nasty disease."

And Professor Dunbar says these emerging diseases have one huge advantage over us: "There will be more Aids-type diseases to come, for ever and ever. That's partly because these small organisms can mutate faster than we can."

Costing the Earth is broadcast on BBC Radio Four at 2100 BST on 19 October.